Conventional wireless communication system use encoders to reduce the amount of bandwidth required for the transmission of speech signals over the air. Typically, the encoder compresses the digitized speech signal into a packet signal having reduced bandwidth. Generally, the encoder analyzes speech signals and designates particular codes to represent particular characteristics in the digital signal. For example, fast changing sounds requiring more information and therefore receive a higher percentage of bits than slow changing sounds.
After compression by the encoder, compressed speech packets are subsequently transmitted through the air over an RF chapel to a base station unit, where the speech packets may be transmitted to another remote base station unit or decompressed and transmitted to a local telephone subscriber.
Although encoders may be used to reduce transmission bandwidth requirements by compressing digitized speech, the encoders may distort voice band tone data, such as dialed digit information, that may be present in the speech signal. Typically, dialed digit information is represented by dual tone multifrequency (DTMF) tones that are generated when a subscriber presses a key on a telephone keypad. Since conventional encoders are designed to compress speech data instead of tone data, compressing a speech signal containing DTMF tones may result in losing at least some of the dialed digit information.
One approach to address this problem has been used by conventional mobile telephone systems using an IS-54 mobile telephone. The IS-54 mobile telephone has a keypass that senses which key is pressed and then sends digit information for the pressed key to the base station unit over a communication channel that is separate from the voice chapel. Typically, the keypass information is transmitted over the control channel and the voice information is transmitted over a traffic channel. The IS-54 telephone also receives digit information over the separate communication channel.
However, there are many applications where a subscriber would rather use a conventional analog telephone instead of a mobile telephone for communication in a wireless system. One such application involves a wireless telephone system allowing a plurality of subscribers in a remote location to receive telephone service. In this application, a radio transmitting unit including an encoder is used to send speech and DTMF digit data over an RF channel to a base station unit that is coupled to a local office within the public telephone network. In such an application, many potential customers would rather use inexpensive traditional analog telephones than the more expensive IS-54 type mobile telephones. In addition, where the radio transmitting unit is physically separated from the subscriber touch tone keypad, a keypress detector similar to those used in IS-54 telephones will not operate correctly.
However, in such an application, a subscriber using a traditional analog telephone will probably not be able to send voice band tones, such as DTMF tones, after establishing a telephone call. The need to send DTMF tones generally arises when the subscriber is communicating with an automated touch tone based system, such as a bank account information system. Since DTMF tones sent by the subscriber's phone are encoded before being transmitted over the air to the base station unit, a DTMF detector at the automated touch tone system will have difficulty detecting the previously encoded DTMF digits.
Thus, it would be desirable to allow a subscriber to use an analog telephone to send voice band tone data instead of using an IS-54 type phone that sends a keypress signal in a wireless voice communication system. Such wireless voice communication systems include but are not limited to fixed wireless systems and airplane air to ground telephone system. Accordingly, there is a need for a wireless voice communication system supporting the use of analog telephones.